1. Field of the Invention
The subject invention relates to light emitting diodes mounted in an array on a circuit and, more particularly, to physically large light emitting diode arrays.
2. Description of the Prior Art
Light emitting diodes (LEDs) have been available since the early 1960's in various forms, and are now widely applied in a variety of signs and message boards. The relatively high efficacy of LEDs (in lumens per Watt) is the primary reason for their popularity. Tremendous power savings are possible when LED lamps are used to replace traditional incandescent lamps of similar luminous output. With the increasing popularity and technology of LEDs, physically large LED arrays are becoming more common as LED performance improves and allows the devices to be used for large outdoor display and advertising panels. Such signs are commonly fabricated with large numbers of high power LEDs (0.5 Watt and higher) which require adequate heat sinking for reasonable life. These LED signs are commonly assembled with wired modules or more preferentially with integral ‘plug and play’ substrates which have all the LEDs interconnected and heat sinked onto a metal plate. U.S. Pat. No. 5,857,767 to Hochstein describes this technology, and it has been widely applied commercially for lighting channel letters, shelter lighting, and high performance beacons and searchlights for the military. As noted in the '767 patent, the conductors that interconnect the LEDs disposed on the insulated metal substrate, are typically screen printed with a conductive ink that is subsequently cured to form the circuit traces. LEDs are then adhesively secured to these printed conductors to complete the circuit.
While this process is very cost effective in larger volumes, the necessity of preparing printing screens is burdensome for small runs, particularly in larger sizes, which can approach several square meters. The circuit artwork is typically computer generated, but actual size, process negatives (or positives} must be created and then ‘burned in’ on a suitably photosensitized screen. That screen must then be developed, washed out and placed in a precision screen printing machine. Conductive, usually silver, based inks are then used to create the circuit (impression) on the insulated metal substrate. When only a few of these substrates are printed, the waste of silver ink is significant, and the process looses its attractiveness because of cost and complexity. It is impractical to use conventional printed circuit techniques for large LED panels because of the cost. Further, photoetched printed circuits employ environmentally unfriendly processes that become unmanageable with very large substrates.